Manufacture of permanent magnets



April 9, 1946. A. UNI- EY TAL 2,398,018

MANUFACTURE OF PERMANENT MAGNETS Filed July 13, 1942 2 Sheets-Sheet lNoEn/Toes aeaffm in Ndmoegnenaamdm April 9, 1946. A. LINLEY ET Al.

MANUFACTURE OF PERMANENT MAGNETS Filed July 13, 1942 2 Sheets-Sheet 2FIGA.

Patented Apr. 9, 1946 MANUFACTURE F PERMANENT MAGNETS Anthony Linley andAlan Tom, Sheield, England Application July 13, 1942, Serial No. 450,746In Great Britain September 26, 1941 9 Claims. (Cl. 175-21) Thisinvention relates to the manufacture of permanent magnets from alloys ofthe precipitation hardening type, particularly those made from an ironbase alloy containing aluminum, nickel and cobalt as the principalalloying elements, its object being to provide an improved process forproducing anisotropic properties therein.

More particularly the object of the invention is to provide an improvedmethod of cooling a magnet alloy of the precipitation hardening type insuch manner that not only is it subjected to the action of a magneticfield whilst cooling from an elevated temperature but such cooling isautomatically YeHected at such varying rates through dierent ranges oftemperature as are conducive to providing optimum conditions forpromoting the desired metallurgical and magnetic changes whereby optimumanisotropic magnetic characteristics are imparted to the alloy.

Another object is to enable asubsequent ageing or tempering process tobe either dispensed with or reduced or the time thereof to beconsiderably reduced.

Other objects of the invention will be apparent from a perusal of thefollowing speciiication.

In the manufacture of magnetic iron and steel castings it has beenproposed, for the purpose of making very dense castings of high magneticconductance, to produce a flux of lines of force in a good magneticconductor and to close the magnetic circuit by the molten iron or steel.

Anisotropic characteristics have however, hitherto been produced inpermanent magnet alloys by a process such as is described, for example,in the British patent specication No. 522,731, in which the alloy iscast, reheated to a high temperature of 1250 C. or thereabouts andwhilst cooling therefrom is subjected to the action of a magnetic iield.Such a process appears to involve a combination oi two physicai changesnamely (a) a metallurgical change in which precipitation or dispersionhardening takes place due to the precipitation of the second phase in acontrolled amount, and (b) a magnetic change in which the magneticdomains existing in the material are re-orlentatedin a definitedirection whilst thematerial is in a condition of internal mobility,thus permitting anisotropic properties to be developed.

Both these changes must occur Itogether during heat treatment ofpermanent magnetic materials to give optimum anisotropic properties.

the alloy from a high temperature (1250 C. or thereabouts) at a definiterate of cooling, which is dependent upon the size of thel article. Thisrate of cooling should be of the order of 1-C.

to 5 C. per second on the average, over a cooling range of from 1200 C.to 600 C. Unavoidably, however, the second or magnetic change (b) mustbe carried out over a certain portion of the temperature range extendingfrom above the Curie point to a temperature about C. below it. Now it isfound that although the averagerate of cooling may be ofthe order of 1C. to 5 C. per second, the cooling should be conducted in two stages.First, at a rate of say. 3 C. to 5 C. per second over the range 1250" C.or thereabouts down to the Curie point and then at a slower rate,preferably at l/2 C. to'3 C. per second or even slower. The mostdesirable rates of cooling are those which will give optimum dispersionhardening conditions in the first range and thereafter will allow asufficient time for eilective application of the magnetic neld, in orderto reorientate the domains.

In practice this differential cooling rate is difficult to obtain in awide variety of sizes and shapes, particularly in conjunction withpreviously known methods of applying the magnetic field by means ofelectro-magnets or permanent magnets.

Further, because of the aforementioned dim- 1 culties in obtaining therequisite cooling rates, it is invariably found necessary, subsequently,in order to achieve optimum precipitation within the alloy, to subjectthe heat treated alloy to anageing or tempering process to bring aboutfurther metallurgical precipitation which has been suppressed by the toorapid cooling rate in the second range.

To obtain precise control of this further precipitation and in order notto disturb the magnetic anisotropy oi'the heat treated alloy, thisageing or tempering process has had to be carried out by-subiectin'g thealloy to long-periods of heating at comparatively low temperatures (say500 C. to 600 C.) well below the critical temperature range of the alloybetween the Curie point and 150 C. below it. The times of heating thealloy at such low temperatures in this ageing or tempering process maybe anything up to 48 hours, depending upon the degree of precipitationalready obtained in the previous combined heat and magnetic treatment. v

Permanent magnet alloys to which this treatment is applied are wellknown for their large grain size and consequent fragility. Thetreatments referred to above hitherto necessary for producinganisotropic properties in these magnetic materials, tend seriously toaffect the mechanical strength and to impair the surface of suchmaterials.

By the method of the present invention not only are the most desirabledifferential rates of cooling to produce anisotropic properties in acast magnetbody of the precipitation-hardening type of alloy obtainedand the time of an ageing process to give optimum results considerablyreduced but the tendency hitherto experienced of such treatments to aectthe mechanical strength and to impair the surface is very muchdiminished, thereby producing anisotropic magnets of better mechanicalstrength and form.

The present invention in its preferred embodiment also contemplates theprovision of novel apparatus for casting a permanent magnet having thedesired anisotropic properties. The apparatus preferably -includes asand mould having moulding or casting spaces therein, and permeablemetallic bodies closely adjacent certain of the faces of the castingspace. These metallic bodies form pole-pieces of a magnetic circuit andthe pieces at opposite sides of the casting space are of oppositepolarity. Alternatively these pole-pieces may comprise bar magnets.

A relatively large runner or sprue opening is provided in the mould, sothat the resulting runner will be of considerable mass so as to coolrelatively slowly. The masses of the pole-pieces, which act as chills,are so chosen that the desired initial and subsequent rates of coolingof the magnet castings are attained.

Other objects and features of novelty will be apparent from thefollowing specific descriptions when read in connection with 'theaccompanying drawings in which certain embodiments of the invention areillustrated by way of example.

In the accompanying drawings:

Figure 1 is a sectional elevation of apparatus according to the presentinvention for producing anisotropic properties in permanent magnets.

Figure 2 is a section on the line 2-2 of Figure 1, being a plan of thelower portion of the moulding box.

Figures 3 and 4 are views similar to Figures 1 and 2` respectively andillustrate a modification.

Like reference numerals indicate like parts Y throughout the drawings.

The apparatus illustrated in Figures 1 and 2 is designed for thesimultaneous manufacture of eight bar magnets and comprises a mouldingbox having an upper portion 5 and a lower portion 6. In the uppersurface of the lower portion 6 of the moulding box two series ofpole-pieces 1 of high permeability material are embedded and arearranged in two parallel rows or series as shown in Figure 2. A mouldingspace 8 for a bar magnet is formed in the sand between each two of thepole-pieces 1 in each row so that each polepiece presents one face to amoulding space 8. These polar faces (of the pole-pieces) may themselvesform the ends of the moulding spaces 8 or a thin layer of sand or othermaterial may intervene.

A casting runner 9 is formed in the upper portion 5 of the moulding boxand a feeder channel I0 is formed between the two series of mouldingspaces 8 with branches leading to each of them.

Each of the pole-pieces 1 has an extension II leading to the base of thelower portion 8 of the moulding box. The extensions II may be integralwith the pole-pieces 1 or they may be provided by separate bodies ofhigh permeability material. The extensions I I rest on a non-magneticsupporting sheet I2.

The two pole-pieces 1 which present faces to each one of the mouldingspaces 8 are connected in a magnetic circuit so that these faces are ofopposite polarity as indicated by the letters N and S and the mouldingspace forms an air-gap in the circuit. The various magnetic circuits aregenerated by electro-magnets each of which comprises a laminated core I3and a winding I4. 'I'he electro-magnets are supported on a mild steelbase plate I5 and they are disposed in two series, corresponding to theseries of pole-pieces 1 with their several cores I8 registering with theseveral pole-piece extensions II. As shown in Figure 1 the cores I3extend through apertures in the non-magnetic sheet I2 to make contactwith the extensions I I. Such contact is, however, not essentialprovided the supporting sheet I2 is suiiiciently thin to enable amagnetic flux density of about 1,000 gauss to be produced across theair-gaps formed by the moulding spaces 8.

It will be appreciated that the mass of the polepieces 1 may be vvariedas desired by selecting bodies to form them of the requisite size andshape, the mass being such lin relation to that of the magnets to becast in proximity thereto as to give the desired initial rate of coolingto the castings. Similarly the feeder channel I0 with its branches maybe varied in capacity to slow down the subsequent cooling of thecastings to the desired rate.

In the modification illustrated in Figures 3 and 4 the pole-pieces 1which have polar faces presented to the moulding spaces 8 are energizedby permanent magnets I6 or by electro-magnets I1 having windings I8. Themagnets I6 and I1 are of the bar type and are vertically's embedded inthe moulding sand as shown with the upper end 'of each magnet makingcontact with a pole-piece 1 and with the lower ends of all the magnetsyoked together by a supporting base-plate I9 of mild steel. Thepole-pieces 1, moulding spaces 8 and magnets I6 and I1 are arranged inparallel rows and adjacent magnets are oppositely disposed as regardspolarity as indicated by the letters N and S so that each of themoulding spaces 8 forms an air-gap in a magnetic circuit.

In Figures 3 and 4 the right hand moulding spaces 8 are shaped for themanufacture of horse-shoe magnets and the remainder for bar magnets andit will be understood that other shapes of magnets maybe manufacturedaccording to the invention by modifying the shape of the moulding spaceand, if necessary, that of the pole-pieces 1 and the disposition andshape of the energizing magnets I8 or I1, I8.

A suitable alloy for manufacturing anisotropic magnets according to theinvention consists of an iron base containing 10 to 20% of nickel, 14 to30% of cobalt, 6 to 11% of aluminium, together with copper up to 7% andtitanium up to 5%. There may be present in the alloy small quantities ofcarbon, silicon and manganese as is usual in permanent magnet alloy,with or without small quantities of tungsten, chromium, columbium,molybdenum, tantalum, vanadium, boron, magnesium, selenium, zirconiumand uranium. In the production of these alloy primary alloys of copperand cobalt may be used.

According to the present invention, therefore,

v anisotropic properties are produced in permanent cipitation-hardeningtype in contact with polar faces of bodies embedded in moulding sand andconnected in a magnetic circuit so that such bodies act also as chillspromoting a rapid initial rate of cooling in the solidifying alloy (ofan average order of from 3 C. to 10 C. per second) and the surroundingsand and heat supply from the casting runner and feeder are utilized topromote a slower rate of cooling as the temperature of the casting fallsto the region of the Curie i point and below.

The masses of the bodies embedded in the moulding sand, of the sand andof the casting runner and feeder are preferably such in relation to thatof the casting itself that the rapid initial cooling rate is reduced toabout 3 C. per second whilst the temperature of the casting is stillwell above the Curie point and that there-A after the retardation of thecooling rate is progressive so that it takes from one quarter to onehalf an hour or longer for the casting to cool to 600 C. and very muchlonger (four hours, for example) to cool to 500 C.

It will be appreciated that in all cases it will be desirable to wait asuicient period of time to permit the casting to cool to 600 C. or lowerbefore removing it from the mould. Preferably the casting is allowed tocool to about room temperature before stripping it and in this mannerthe combined influence of thel favourable differential cooling ratesobtained whilst the casting is cooling down to about 600 C. and therelatively much longer time and slower cooling rate involved between 600C. and 500 C. or lower will render a subsequent ageing or temperingprocess unnecessary. In any case the length of time required in anysubsequent ageing or tempering process will be drastically curtailedbeing inversely dependent on the degree of precipitation obtained in themould.

In a particular example of manufacturing anisotropic permanent magnetsaccording to the invention an iron base alloy containing 15% of nickel,7.5% of aluminium, 25% of cobalt, 2.5% of copper and 1% of titaniumwascast in the apparatus illustrated in Figures 3 and 4 and the magnetswere removed from the moulds when cool.

On testing the magnets they were found to possess the following magneticproperties:

B. Rem gauss 11,500 Hc oersted 380 (BHhnax 2.4 10

After an ageing o r tempering process of one hour at 600 C. the resultswere:

B. Rem gauss- 10,400v Hc oersted 660 (BH) max 3.46 X 10 Material of thesame composition when cast in the ordinary manner, reheated and cooledin a magnetic field from 1250 C. gave the following results:

B. Rem gauss 11,750 Hc oersted-- 310 (BH) max 1.8 X 10 After an ageingor tempering process of one hour at 600 C. the magnets possessed thefollowing properties:

B. Rem gauss 10,800 Hc oersted 620 (BHhnax 3.18 X 10s After an ageing ortempering process of 15 hours at 600 C. the values were found to be:

B. Rem gaus's-- 11,1oo Hc oersted-- l650 (B1-nw 3.32 10 It will be seentherefore that by the employment of the present invention the periodrequired in the ageing' or tempering process to give optimum results maybe reduced from 15 hours to 1 hour.

What we claim is:I

1. The process of producing a permanent'magnet having desirableanisotropic properties, which comprises casting, with a.relatively largecasting runner in a mould, a quantity of a magnet alloy oi' theprecipitation-hardening type in contact with polar-faces of metallicbodies connected in a magnet circuit, whereby said bodies promote arapid initial rate of cooling in the solidifying alloy and whereby theheat .if the runner is utilized to effect a slower rate of cooling asthe temperature of the casting falls further.

2. The process of producing a permanent magnet having desirableanisotropic properties, which comprises casting, with a relativelylargecasting runner in a sand mould, a quantity of a magnet alloy oi'the precipitation-hardening type in contact with polar faces of metallicbodies connected in a magnet circuit, whereby said-bodies promote arapid initial rate of cooling in the solidifying alloy and whereby theheat of the runner and the insulating properties of the surrounding sandare utilized to eifect a slower rate of cooling as the temperature ofthe casting 'falls further.

3. The process of producing a permanent magnet having desirableanisotropic properties, which comprises casting, with a relatively largecasting runner in a sand mould, a quantity of a magnet alloy of theprecipitation-hardening type in contact with polar faces of metallicbodies connected in a magnet circuit, whereby said bodies promote arapid initial rate of cooling in the solidifying -alloy and whereby theheat of the runner and the insulating properties ofthe surrounding sandare utilized to effect a slower rate of cooling as the temperature ofthe casting falls to the region of the Curie point and below.

4. The process of producing a permanent magnet having desirableanisotropic properties, which comprises casting, with a relatively largecasting runner in a mould, a quantity of a magnet alloy of theprecipitation-hardening type in contact with polar faces of metallicbodies connected in a magnet circuit, whereby said bodies promote arapid initial rate of cooling in the solidifying alloy and whereby theheat of the runner is utilized to eil'ect a slower rate of cooling asthe temperature of the casting follows further to the region of theCurie point and below,and thereafter subjectingthe casting to an ageingor tempering process in which it is held at a tempera- 'ture of from 500C. to 600 C. for a period inversely dependent on the degree ofprecipitation obtained in the mould.

5. The process of producing a permanent magnet having desirableanisotropic properties, which comprises casting, with a relatively largecasting runner in a mould. a quantity of a magnet alloy of theprecipitation-hardening type consisting of an iron base containingaluminium, nickel, and cobalt as the principal alloying elements incontact with polar faces of metallic bodies connected in a magnetcircuit, whereby said bodies 'promote a rapid initial rate of cooling inthe solidifying alloy and whereby the heat of the runner is utilized toeffect a slower rate of cooling as the temperature of the castingfollows further to the region of the Curie point and below.

6. A permanent magnet possessing anisotropic properties and producedaccording to the process set forth in claim l.

7. Casting apparatus for producing permanent magnets having anisotropicproperties comprising a mould having a moulding space therein, metallicbodies embedded in the mould and each presenting a face to the mouldingspace, means providing a magnetic circuit including the pole pieces, arelatively large runner space connecting the pouring opening to themoulding space, whereby an initial rapid rate of cooling of the castingis accomplished by the chill effect of the metallic bodies, and asubsequent slower cooling is eil'ected by the slower cooling of therelatively large casting runner.

8. Casting apparatus for producing permanent magnets having anisotropicproperties comprising a sand mould having a moulding space therein,metallic bodies embedded in the sand mould and each providing oppositesurfaces of the moulding space, means providing a magnetic circuitincluding the pole pieces, a relatively large runner space connectingthe pouring opening to the moulding space, whereby an initial rapid rateof cooling ot the casting is accomplished by the chill effect of thepole pieces, and a subsequent slower cooling is etl'ected by the slowercooling of the relatively large casting runner within the sand mould.

9. Casting apparatus for producing permanent magnets having anisotropicproperties comprising a sand mould having a moulding space therein, polepieces of high permeability embedded in the sand mould and eachproviding opposite surfaces of the moulding space, means providing amagnetic circuit including the pole pieces whereby the latter are ofopposite polarity, a relatively large runner space connecting thepouring opening to the moulding space, whereby an initial rapid rate ofcooling of the casting is accomplished by the chill effect of the polepieces, and a subsequent slower' cooling. is eected by the slowercooling of the relatively large castingrunner within the sand mould.

` ANTHONY LINLEY.

ALAN TORRY.

